As the name suggests the recovery section was installed to recover the cooking chemicals from the Black liquor received from the Brown Stock Washing section. Although, this section is much cost-intensive it needs to be installed to avoid the use of 100% fresh cooking chemicals. As the losses in the Brown Stock Washing is 8-12%, the requirement of the fresh chemicals will be only 8-12%.

The main subheads in the Recovery plant are,

• Multiple Effect Evaporators
• Low-Temperature Incinerator
• Recaustisicing Plant

Multiple Effect Evaporator Section

The Black Liquor received from Brown Stock Washing Section is having a concentration of 0.8- 1.5% depending upon the use of Raw Material. The Concentration of Black Liquor is increased to 25-30% in the evaporator section. There are different types of evaporators being used in the paper industry depending upon the quality of Black Liquor. These are Long Tube Vertical evaporators, Falling Film Evaporators, Natural Circulation Evaporators, etc.

The most common evaporator being used in the paper industry are Falling Film Evaporators which are having advantages over Long Tube Vertical Evaporators, which are as follows.

• In Falling Film Evaporator, the black Liquor is pumped from the Vapour Head to the Distribution Plates located above the Tubes allowing the liquor to fall down within the tubes.
• The Falling Film Evaporator working at a low Temp. Difference and which allows many effects to be installed in series. It also has a better steam economy than other types of evaporators.

Low-Temperature Incinerator

The basic concept of this system is the use of fluidized bed technique for the treatment of black liquor received from Multiple Effect Evaporator to recover the Pulping chemicals as well as to reduce the pollution load on the environment. The Low-Temperature Incinerator is used in the paper mill, where the Black Liquor solids are less than 300 tons per day.

IN LTI, The solids in the liquid waste containing heat energy are amenable. The advantage of LTI is,

• Elimination of Air as well as Water Pollution.
• Removal of BOD & color, etc. from residual organic products.
• Removal of all particulate matter & gases before discharged into the atmosphere.

The system consists of mainly

• Low-Temperature Incinerator Proper With Accessories
• Venturi Scrubber System with Cyclone Separator
• Attrition Mill and other Accessories
• Air Pre-heater with HSD Burner

The Semi Concentrated Black Liquor from the evaporator is fed to Venturi Scrubber, which works on the Direct Contact Evaporator. The Concentration of Black Liquor is increased to 45%. This Black Liquor is fed to LTI- where the Temp. Is around 600-700 Deg. C is maintained. The water in the Black Liquor is evaporated & the solids are converted to Soda Ash.

The Soda Ash produced is withdrawn from the side of the reactor at a controlled rate by a variable speed Rotary Valve. The Soda Ash is conveyed through Water Cooled Screw Conveyor before being stored in a Silo.

Recaustisizing Plant

The Soda Ash (NA2CO3) produced in the LTI- is further treated with milk of lime (Ca(OH)2) to get Caustic Soda (NaOH) which is used in the Digester for the Cooking of the Raw Material & the Calcium Carbonate (CaCO3) produced is either used as Landfills or sold to Cement Industry.

The Major Equipment in Recaustisicing Section is as below.

• Lime Slurry Preparation
  • Table Feeder: A table Feeder is installed to feed the lime to slaker at a controlled rate.
  • Rotary Drum Slaker: In Drum Slaker, the water is added along with the lime to make the slurry. The slaker rotates at a fixed speed & required time is given to complete the reaction. The grits in the lime are removed from the slaker.
  • Classifier: The green liquor from LTI Section is mixed with Lime Slurry in Rake Classifier. Any grits in the system are removed in the Classifier.
  • Causticizer: The Slurry from Rake Classifier is pumped to Causticizer, which is installed in series to provide the required retention time to complete the reaction. The Causticizer has an agitator & the size depends on the size capacity.
  • White Liquor Clarifier: The slurry from Causticizer is pumped to White Liquor Clarifier, which is provided with a rake mechanism having blades to ring the sludge in the center. The Overflow from the clarifier is taken to the Storage Tank, from where it is pumped to the Digester House to be used as Cooking chemicals.
  • Recausticizers: The underflow from White Liquor Clarifier, is taken to Recausticizers- where it is mixed with Washed Liquor from Ist stage Lime Mud Washer.
  • Lime Mud Washer: Lime Mud Washers are similar to White liquor Clarifier, which are provided with Rake Mechanism. The Lime Mud Washers are provided in Series to recover the chemicals from the Lime Sludge & the wash water is used in the Counter-Current system.
  • Lime Mud Filter: The Sludge from the final Lime Mud Washer is pumped to Lime Mud Filter, which works on the vacuum. The inlet consistency of lime sludge is around 1-1.5% and outlet consistency is around 60%. The lime sludge from the lime mud filter can be used either in landfills can be sold. Lime Mud Filter is provided with a filtrate tank.